This invention relates to the storage of information, and, more particularly, to an erasable memory device utilizing an amorphous metallic storage medium.
Computers and other types of information processing equipment have become indispensable to the modern industrial world. One of the key areas of continuing development of computers is the search for information storage systems, or memories, that can store ever larger amounts of information. A wide variety of types of memories are available, including random access memories and bulk storage devices.
More existing memories are based upon electrically induced magnetic transformations, wherein the magnetic state of a magnetic medium is locally transformed in a controllable pattern to store information in the pattern. The pattern of transformed and untransformed areas can be deduced by sensing whether particular areas are in one or the other of the possible magnetic states. Magnetic memories are widely used in one form for random access memory, and in other forms for bulk storage devices such as magnetic tapes and magnetic disks.
The total amount of information that may be stored in a magnetic memory is limited by the minimum size of a magnetically measurable area of a magnetic medium. Other types of storage devices have been proposed, as the demands for greater information storage have exceeded the ability to create and measure small magnetic regions.
Optical storage systems are now available commercially in a read-only form. Such optical information storage systems have readable patterns of pits or other irregularities in an otherwise smooth surface. The patterns are placed into the surface at a central location by a highly focused laser. The pattern of pits can be read by another laser because of the lower optical reflectivity of the irregularity as compared with the otherwise high reflectivity of the surface in which the pits are made. However, once the pattern of pits is made, there is no practical method for transforming the pit back to a smooth, highly reflective surface, effectively erasing the prior information so that new information can be stored in the same area.
More recently, erasable optical memory systems have been under development. In these systems, a material whose optical reflectivity is controllable and changeable between two states is coated onto a substrate. Information is stored by a system similar in many respects to that of the read-only optical memory system. However, the transformable material remains essentially smooth on the surface of the substrate, and only its reflectivity is changed, not its physical form. The state of a region is interrogated by an optical beam, whose reflection is determined by the state of the region. From this interrogation, stored information is reconstructed from the pattern of the optical states.
One material frequently considered for such erasable optical memory systems is doped tellurium oxide, which is deposited on a substrate in a thin layer, as about 0.07 microns thick. An acrylic layer is coated over the tellurium oxide to protect it from oxidation or other external influence that might change its reflectivity, apart from intentionally induced changes. A writing and erasing laser transforms a region of the layer between the amorphous and the crystalline states. The reflectivity of the tellurium oxide is different in these two states, and this difference can be read as the difference in reflectivity of the beam of an interrogating low power laser, which beam is reflected from the surface. Because the reflectivity difference involves changes in the crystalline state of the tellurium oxide in the layer, rather than the physical form of the layer, and because the transformation between the amorphous and crystalline states is reversible, information can be stored in the layer, read, and later erased, so that new information can be written into the same area.
While the erasable optical memory, once perfected, will be an imporatant advance in memory systems, there remain problems that limit its usefulness. The surface of the medium must be carefully controlled so that the reflectivity does not change due to extraneous factors. Minor warping of the surface due to heat or stresses could render it ineffective. The tellurium oxide has limited stability against damage induced by temperature changes and radiation, and the transformation between the amorphous and crystalline states can be affected by numerous external influeneces. The composition of the memory material and the structure of the protective coating are critical. Thus, at least for the foreseeable future such erasable optical memory systems will be highly sensitive to external influences, and will be used only in carefully controlled environmental conditions.
There are many applications where high density erasable memories are subjected to heat, stress, and difficult environmental conditions, as in spacecraft. Although attempts may be made to retain a stable environment, it would be desirable to have such a memory storage device that is stable in the presence of adverse environmental influences such as heat, radiation, and magnetism, and wherein the stored information is not rendered unretrievable by environmental fluctuations.
A need therefore exists for such a stable, high density, erasable memory storage device. The present invention fulfills this need, and further provides related advantages.